The present invention relates to a method of producing an optical blank from a crystalline material, as well as to an optical blank. The optical blank serves as a preliminary stage in the production of a lens or a lens part. Consequently, the invention also relates to a method of producing a lens or a lens part from a crystalline material, as well as to a lens or lens part. Lenses or lens parts of the kind that the invention relates to are used in optical objectives, specifically in projection objectives for a microlithography projection system. Consequently, the invention also relates to objectives, specifically projection objectives for a microlithography projection system.
Methods for producing an optical blank from a fluoride crystal material have been disclosed in U.S. Pat. No. 6,201,634. The optical blank is used to make lenses for a projection objective for a microlithography projection system. The lens axes of the lenses are oriented with preference in the crystallographic <111>-direction. According to U.S. Pat. No. 6,201,634, the crystallographic <111>-direction is selected for the purpose of minimizing the detrimental effects of stress-induced birefringence.
It is a general trait of birefringent lenses that a non-polarized light ray is split into two rays with, respectively, different states of polarization, different speeds of propagation, and different directions. When used in an objective, birefringent lenses will cause a loss in optical resolution unless appropriate corrective measures are taken. The birefringent effect in lenses can be caused, for example, by stress-induced birefringence that occurs as a result of the manufacturing process or as a result of mechanical forces acting on the lens. The phenomenon of birefringence is of particular importance in crystal optics. Anisotropic crystals are birefringent. However, isotropic crystals, too, such as the cubic fluoride crystals, exhibit an intrinsic birefringence, which becomes particularly noticeable at wavelengths in the far ultraviolet range (<200 nm). Cubic fluoride crystals such as calcium fluoride and barium fluoride are preferred lens materials for projection objectives with a working wavelength in this range. Consequently, in view of its detrimental effect at wavelengths in the far ultraviolet range, the intrinsic birefringence of these crystals needs to be compensated by appropriate measures.
In the present context, it is essential to have a clear system of notations for the crystallographic directions. Following are the notations by which crystallographic directions, crystallographic planes, and lenses whose lens axes are aligned in a specific crystallographic direction will hereinafter be characterized.
The indices for the crystallographic directions will hereinafter be bracketed between the symbols “<” and “>”, and the indices for the crystallographic planes will be bracketed between the symbols “{” and “}”. The crystallographic directions are perpendicular to the correspondingly indexed crystallographic planes. For example, the crystallographic direction <100> is perpendicular to the crystallographic plane {100}. Crystals with a cubic lattice structure, which includes the fluoride crystals that are of interest in the present context, have the principal crystallographic directions <110>, < 110>, <1 10>, < 1 10>, <101>, <10 1>, < 101>, < 10 1>, <011>, <0 11>, <01 1>, <0 1 1>, <111>, < 1 1 1>, < 1 11>, < 11 1>, <1 1 1>, < 111>, <1 11>, <11 1>, <100>, <010>, <001>, < 100>, <0 10>, and <00 1>.
Because of the symmetries of cubic crystals, the principal crystallographic directions <100>, <010>, <001>, < 100>, <0 10>, and <00 1> are equivalent to each other. Therefore, those crystallographic directions that are oriented along one of the principal directions <100>, <010>, <001>, < 100>, <0 10>, and <00 1> will hereinafter be identified by the prefix “(100)-”, and crystallographic planes that are perpendicular to these directions will also be identified by the same prefix “(100)-”.
Furthermore, the principal directions <110>, < 110>, <1 10>, < 1 10>, <101>, <10 1>, < 101>, < 10 1>, <011>, <0 11>, <01 1>, and <0 1 1> are likewise equivalent to each other. Therefore, those crystallographic directions that are oriented along one of the latter group of principal directions will hereinafter be identified by the prefix “(110)-”, and crystallographic planes that are perpendicular to these directions will also be identified by the same prefix “(110)-”.
Finally, the principal directions <111>, < 1 1 1>, < 1 11>, < 11 1>, <1 1 1>, < 111>, <1 11>, <11 1 > are likewise equivalent to each other. Therefore, those crystallographic directions that are oriented along one of the latter group of principal directions will hereinafter be identified by the prefix “(111)-”, and crystallographic planes that are perpendicular to these directions will also be identified by the same prefix “(111)-”.
Any statements made hereinafter in regard to one of the aforementioned principal crystallographic directions should be understood to be equally applicable to the equivalent principal crystallographic directions.
As is known from the article “Intrinsic Birefringence in Calcium Fluoride and Barium Fluoride” by J. Burnett et al. (Physical Review B, Volume 64 (2001), pp. 241102-1 to 241102-4), lenses made of calcium fluoride crystal material or barium fluoride crystal material exhibit intrinsic birefringence. The intrinsic birefringence is in this case strongly dependent on the material orientation of the fluoride crystal lens and on the light ray direction. The effect is maximal for a ray that passes through a lens along the crystallographic (110)-direction. The measurements presented by Burnett et al. demonstrate that a light ray traveling in the crystallographic (110)-direction of a calcium fluoride crystal is subject to a birefringence that amounts to 11.8±0.4 nm/cm at a wavelength of λ=156.1 nm, to 3.6±0.2 nm/cm at a wavelength of λ=193.09 nm, and to 0.55±0.07 nm/cm at a wavelength of λ=253.65 nm. On the other hand, with a light propagation in the <100> direction or in the <111> direction of the crystal, no intrinsic birefringence occurs in calcium fluoride, as is also predicted by theory. Thus, the intrinsic birefringence has a strong directional dependence and increases significantly for shorter wavelengths.
The directional dependence of the intrinsic birefringence in a fluoride crystal with a cubic crystal structure is shown in the published article “The trouble with calcium fluoride” by J. Burnett et al. (spie's oemagazine, March 2002, pp. 23-25 and FIG. 4), which may be accessed at “http://oemagazine.com/fromTheMagazine/mar02/biref.html”. The intrinsic birefringence of a light ray depends in this case on the aperture angle as well as on the azimuth angle of a light ray. As is made evident in FIG. 4, the intrinsic birefringence has a fourfold azimuthal symmetry if the lens axis is oriented in the crystallographic (100)-direction, a threefold azimuthal symmetry if the lens axis is oriented in the crystallographic (111)-direction, and a twofold azimuthal symmetry if the lens axis is oriented in the crystallographic (110)-direction. By rotating two fluoride crystal lenses relative to each other about their lens axes, it is possible to reduce the detrimental influence of the intrinsic birefringence. Favorable results are obtained with an angle of rotation of 45° for two lenses whose lens axes are oriented in the crystallographic (100)-direction, with an angle of rotation of 600 for two lenses whose lens axes are oriented in the crystallographic (111)-direction, and with an angle of rotation of 900 for two lenses whose lens axes are oriented in the crystallographic (110)-direction. By simultaneously using respective pairs of (100)-lenses, (111)-lenses, and (110)-lenses, it is possible to reduce the optical path difference between two mutually orthogonal states of polarization. As a further possibility, using calcium fluoride lenses and barium fluoride lenses in combination likewise results in a compensation of the detrimental influence of the intrinsic birefringence because, according to FIG. 2 of the same article, the respective birefringence effects for corresponding crystallographic directions in barium fluoride and calcium fluoride have opposite signs.
Projection objectives and microlithography projection systems have been disclosed, e.g., in the Patent Application Publication WO 01/50171 A1 (U.S. Ser. No. 10/177,580) and the references cited therein. The examples of embodiments presented in that patent application are purely refractive as well as catadioptric projection objectives with numerical aperture values of 0.8 and 0.9 at working wavelengths of 193 nm as well as 157 nm. The material used for the lenses is calcium fluoride.
The not pre-published patent application PCT/EP 02/05050 (U.S. Ser. No. 10/367,989) by the same applicant includes a description of different compensation methods for reducing the detrimental influence of the intrinsic birefringence, e.g., in the objectives that are presented as examples in WO 01/50171 A1 (U.S. Ser. No. 10/177,580). Among other possibilities, the solutions disclosed therein include the parallel use of (100)-lenses with (111)-lenses or (110)-lenses of the same fluoride crystal material as well as the use of compensation coatings. The disclosures of PCT/EP 02/05050 (U.S. Ser. No. 10/367,989) and of WO 01/50171 A1 (U.S. Ser. No. 10/177,580) are hereby incorporated herein in its entirety.
To conclude, the compensation methods described above for reducing the detrimental influence of the birefringence are based among other things on the use of lenses that are rotated relative to each other about their lens axes. The angle of rotation between two lenses depends for example on the crystallographic direction in which the lens axis is oriented. For example in lenses made by a method according to the previously cited reference U.S. Pat. No. 6,201,634, the lens axes are oriented in the crystallographic (111)-direction. Based on what has been said above, a favorable result in reducing the detrimental influence of the intrinsic birefringence is obtained in this case with an angle of rotation of 60° between two (111)-lenses. The angle of rotation is defined in relation to the crystallographic structures of the two lenses. However, the outward appearance of a lens gives no indication of its crystallographic structure.